Alveolar epithelial cells in patients with fibrotic lung disorders are known to undergo a metaplastic or cuboidal transformation. This study presents the first detailed description of the morphology of these cuboidal cells, and it also provides a hypothesis to explain the formation of such cells. It is concluded that two types of cuboidal cells can be recognized, that they are derived from bronchiolar epithelial cells, and that the latter cells become sources of epithelial renewal in severely damaged alveoli in fibrotic lungs.